In wireless cellular communications systems, one or more base stations that are connected to one or more communications networks support communication of a mobile station with the one or more communications networks by way of a wireless communication channel between the mobile station and the base station.
In an effort to improve spectral efficiency in such wireless cellular communications systems, a digital wireless transmission method called Code Division Multiple Access (CDMA) may be employed. In CDMA, signals for transmission are encoded using one of several pseudo-random sequences that the receiver also knows and can use to decode a received signal. Each of the pseudo-random sequences corresponds to one of several different communication channels.
In further efforts to improve spectral efficiency, an architecture has been devised in which a given base station, while cooperating with several other base stations to supply carriers that define a first coverage area, supplies a carrier to define a second coverage area underlying the first coverage area. A wireless communication channel between a mobile station and the given base station using a frequency associated with the second coverage area may be called an underlying carrier. Additionally, the wireless communication channel between a mobile station and one or more base stations (including the given base station) using a frequency associated with the first coverage area may be called an overlay carrier.
Handoff is a mechanism in wireless cellular communications systems that provides for the transfer of an ongoing call from a first wireless communication channel to a second wireless communication channel. The first wireless communication channel and the second wireless communication channel may, on one hand, both be associated with one broad coverage area or, on another hand, first wireless communication channel may be associated with an overlaying coverage area (with associated overlay carriers) and the second wireless communication channel may be associated with an underlying coverage area (with associated underlying carriers). Each handoff may require network resources to route the call from the base stations supporting the first wireless communication channel to the base stations supporting the second wireless communication channel. In some instances, the same base station supports both wireless communication channels.
“Hard handoff” is a term used in CDMA systems to describe a handoff in several instances. For example, a handoff involving a change from a wireless communication channel using one frequency to wireless communication channel using another frequency may be called a hard handoff. Additionally, a handoff involving a system change, e.g., from a wireless communication channel with the base stations supplied by one manufacturer to a wireless communication channel with the base stations supplied by another manufacturer, may be called a hard handoff. Further, a hard handoff may involve a handoff from a wireless communication channel with the base stations controlled by one base station controller to a wireless communication channel with the base stations controlled by another base station controller.
In a hard handoff, the existing wireless communication channel between the mobile station and the initial base stations is broken before a new wireless communication channel between the mobile station and the final base stations is established. Such a hard handoff is generally used where the wireless communication channel between the mobile station and the initial base stations and the wireless communication channel between the mobile station and the final base stations do not use the same radio frequency channel. Notably, where the hard handoff is from an overlay carrier to an underlying carrier, the initial and final base stations may be the same base stations.
A hard handoff contrasts with a soft handoff, which is generally used when the existing wireless communication channel between the mobile station and the first base station uses the same radio frequency channel as wireless communication channel between the mobile station and the second base station. In a soft handoff, the existing wireless communication channel between the mobile station and the first base station is maintained while a new wireless communication channel between the mobile station and the second base station is established.
A hard handoff may be network-controlled or mobile-controlled handoff and is typically triggered through the recognition that a measured round trip delay (RTD) has exceeded a threshold. RTD is a measure of the time taken for an electronic message to travel from the base station to the mobile station and back to the base station.
However, using RTD to trigger hard handoffs results in an effective coverage area that is circular in shape for a carrier supplied by an omnidirectional antenna at a given base station (i.e., an “omni” base station as distinct from a “sectored” base station with directional antennas). Notably, this circular effective coverage area may be much smaller that the effective coverage area would be if defined according to alternative metrics. The overlay carriers provided by a given omni base station may be considered to have significant unused capacity beyond that in use within the circular effective coverage area. Clearly, an improved trigger for hard handoff is required to use more of the capacity of the overlay carriers.